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Fri May 25 14:11:39 EDT 2018

Signals

To make this work, it is necessary to first understand what Signals
are.  Stick to MyHDL as basic semantics.

Somehow I don't really see why signals need to be so complex.

Maybe just stick to what MyHDL does?

At the very least, I need to distinguish between intermediates (wires)
and registers.

I think I understand.  The difference is whether something appears in
a combinatorial or a sequential block.

So we have some context in which we can do bindings.

Semantics of signals:
1) exactly one driver
2) from comb creates a wire that cannot have loops
3) from seq creates a register which can create loops across clock ticks


Let's try to make this basic program work:

  counter a b = do
    comb $ do
      a <-- not b
    seq $ do
      b <-- b `add` 1

Two blocks, one combinatorial and one sequential, using a uniary and a
binary operation.

Maybe signals can be implicit?
The do notation's <- can be used to create combinatorial signals
Then a 'set' operation can assign a combinatorial signal to a register.


Following through, this code

  counter b = do
    a  <- inv b
    b' <- add b $ L 1
    set b b'
  
  main = do
    printl $ mapToList $ compile $ signal >>= counter

leads to the following network structure:
signal n is driven by driver d.

(n,d)

(0,Reg (Signal 2))
(1,Comb1 INV (Signal 0))
(2,Comb2 ADD (Signal 0) (L 1))

The combinatorial ones are straightforward.  The first one says that
signal 0 is driven by a register, who's input is driven by signal 2.

I'm going to rename Reg to Delay, and separate out constants so they
are explicit drivers of signals:

(0,Delay 3)
(1,Comb1 INV 0)
(2,Const 1)
(3,Comb2 ADD 0 2)


Making a next iteration where signals are explicitly driven, allowing
both explicit combinatorial and sequential drive.



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